1. Field of the Invention
The present invention relates to a dielectric filter and dielectric duplexer which have a plurality of inner-conductors inside a dielectric block, an external conductor disposed on the outer surface of the dielectric block and input-output electrodes disposed on the outer surface of the dielectric block and capacitance-coupled to inner-conductors, and to a communication apparatus including the same.
2. Description of the Related Art
A dielectric filter having, for example, the construction shown in FIG. 7 has been disclosed (Japanese Utility Model Application No. 6-2802). FIG. 7 is a sectional view passing through two input-output electrodes. In this dielectric filter, three inner-conductor holes 2a, 2b, and 2c passing through between a pair of opposing end surfaces of a dielectric block 1 in the form of a nearly rectangular solid are provided, and inner-conductors 3a, 3b, and 3c are formed on respective internal surfaces thereof. At fixed locations on the external surface of the dielectric block 1 a pair of input-output electrodes 5a and 5b are formed, and on nearly the whole of the external surface, except the areas where the input-output electrodes 5a and 5b are formed, an external electrode 4 is formed. This dielectric filter is externally coupled via external coupling capacitances C1 and C2 generated between the input-output electrodes 5a and 5b and the inner-conductors 3a and 3c, respectively, which oppose the electrodes. Each of the inner-conductor holes 2a, 2b, and 2c is formed as a straight hole of the same axis having a constant inner diameter, and, of the three inner-conductor holes 2a, 2b, and 2c, the inner-conductor holes 2a and 2c acting as an input-output stage are arranged close to the sides where the input-output electrodes 5a and 5b are formed.
In this way, without allowing the Q0 valve of a resonator to fall by arranging an inner-conductor hole opposed to an input-output electrode to be close to the input-output electrode, a large external coupling capacitance can be obtained through an input-output electrode with a relatively small area.
However, in the above conventional dielectric filter, as the whole inner-conductor holes acting as input-output stages are arranged close to the side surfaces where the input-output electrodes are formed, there is a problem of the Q0 valve of the resonator being degraded. This is because the best Q0 valve of the resonator is obtained when the inner-conductor hole is arranged in the middle of the dielectric block and the Q0 valve deteriorates further when the hole is set further from the middle.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric filter and dielectric duplexer having a large external coupling capacitance and a higher Q0 valve, and to provide a communication apparatus including the same.
One preferred embodiment of the present invention provides a dielectric filter comprising: a dielectric block;
a plurality of inner-conductor holes each having an inner-conductor disposed on an inner surface thereof provided in the dielectric block; an external conductor disposed on an outer surface of the dielectric block; and an input-output electrode disposed on the outer surface of the dielectric block and capacitance-coupled to the inner-conductors; wherein each of the inner-conductors has an open end in at least one opening surface of the inner-conductor hole or in the vicinity of the opening surface; the inner-conductor hole being opposed to the input-output electrodes has a stepped portion to define a first inner-conductor hole portion and a second inner-conductor hole portion divided thereby; and the first inner-conductor hole portion has the open end and located closer to the input-output electrode than the second inner-conductor hole portion.
In the above described dielectric filter, the sectional area of the first inner-conductor hole portion may be larger than that of the second inner-conductor hole portion.
Another preferred embodiment of the present invention provides a dielectric duplexer comprising at least two filter portions formed in a dielectric block, wherein at least one of the filter portions is composed of the above described dielectric filter.
Yet another preferred embodiment of the present invention provides a communication apparatus comprising at least one of the above described dielectric filters and the dielectric duplexer.
In a dielectric filter or dielectric duplexer having the above construction, as the open end of the inner-conductor hole opposed to an input-output electrode (first inner-conductor hole portion) is arranged so as to be near the input-output electrode, that is, as only part of the inner-conductor hole is made to be eccentric so as to be close to the input-output electrode, a large external coupling capacitance can be obtained, and the Q0 valve is less reduced. That is, when compared with conventional examples where the whole inner-conductor hole is made near an input-output-electrode, the Q0 valve of the resonator can be less reduced.
Furthermore, when the first inner-conductor hole portion is formed so as to have a larger diameter than the other portion (second inner-conductor hole portion), the external coupling capacitance can be further increased.
Therefore, a dielectric filter and dielectric duplexer having a small insertion loss and suitable characteristics can be obtained.
Further, a communication apparatus according to the present invention is composed of a dielectric filter or dielectric duplexer having the above features and accordingly exhibits suitable characteristics.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.